Applications of the density functional method combined with the electron-positron correlation-polarization potential to positron binding to hydrocarbons and water clusters

We present benchmark calculations using the electron-positron correlation-polarization potential (CPP) method with the atomic polarizability model to evaluate the positron affinities of key categories of polyatomic hydrocarbons and water microclusters. The universal model parameter of the generalized gradient approximation for CPP is optimized based on the experimentally measured positron affinities of the representative hydrocarbon molecules. Using this method, the positron affinities and positron binding features of water clusters up to the hexamer are investigated. The present CPP calculations revealed reasonable size-dependence on positron binding energies and conformer-dependent properties for each cluster size for these water clusters. The bound positrons are also trapped internally within cavities of the three-dimensional water clusters, similar to the behavior of bound excess electrons in water cluster anions. However, since the positronic binding energies are smaller than the electronic binding energies in water cluster anions, the bound positrons exhibit delocalized features extending into the electronegative oxygen atoms of the water molecules acting as the multiple hydrogen donor. Such behavior differs from the water cluster anions, resulting in isomeric conformational dependence.